Nuclear spectroscopy corrections using multiple standards and spatial regions

Abstract

A method for estimating an aspect of a formation using a nuclear spectroscopy tool includes placing a nuclear spectroscopy tool including a neutron source and a gamma ray detector into a borehole and performing a plurality of environmental measurements. Neutrons are emitted from the nuclear spectroscopy tool such that some of the neutrons generate gamma rays from a formation adjacent the nuclear spectroscopy tool, some of the neutrons generate gamma rays from elements within the nuclear spectroscopy tool and some of the neutrons generate gamma rays from an element in the drilling mud. An energy spectrum of gamma rays induced by the emitted neutrons can be detected with the tool and analyzed using a combination of standard spectra including at least two sub-standards that represent a common element or group of elements and that are differentiated based on location of neutron interaction, such as where the neutrons thermalize.

What is claimed is: 
     
       1. A method for estimating an aspect of a formation using a nuclear spectroscopy tool, comprising:
 placing a nuclear spectroscopy tool including a neutron source and at least one detector for gamma rays into a borehole; 
 emitting neutrons from the nuclear spectroscopy tool into the formation adjacent the tool such that some of the neutrons can generate gamma rays from the formation, some of the neutrons can generate gamma rays from an element in the nuclear spectroscopy tool, and some of the neutrons can generate gamma rays from an element in a drilling mud; 
 detecting with the tool an energy spectrum of gamma rays induced by the emitted neutrons; 
 analyzing the detected gamma ray spectrum using a combination of standard spectra including at least two sub-standards that represent a common element or group of elements and that are differentiated based on distinct locations of neutron interaction; 
 
       and
 estimating an aspect of the formation using the analyzed detected gamma ray spectrum, wherein the nuclear spectroscopy tool comprises a spatial heterogeneity in its material composition and wherein the nuclear spectroscopy tool comprises a wireline tool having a bow spring extending through or along the wireline tool or a logging-while-drilling tool that is configured to accommodate passage of drilling mud. 
 
     
     
       2. The method of  claim 1 , wherein the neutron source comprises an electronic neutron generator or a chemical source. 
     
     
       3. The method of  claim 1 , wherein the neutron interaction is with the formation, an element in the tool and/or an element in the drilling mud in an inelastic or radiative capture interaction. 
     
     
       4. The method of  claim 1 , wherein the at least two sub-standards represent a combination of elements from a tool background. 
     
     
       5. The method of  claim 4 , wherein the at least two sub-standards represent gamma ray spectra resulting from neutrons thermalizing in distinct portions of the nuclear spectroscopy tool. 
     
     
       6. The method of  claim 5 , wherein the nuclear spectroscopy tool comprises a logging-while-drilling tool, and the at least two sub-standards represent, respectively, gamma rays generated by neutrons that thermalized within the nuclear spectroscopy tool and gamma rays generated by neutrons that thermalized outside of the nuclear spectroscopy tool. 
     
     
       7. The method of  claim 6 , wherein the at least two sub-standards comprise a first sub-standard representing gamma rays generated by neutrons that thermalized within drilling mud inside a flow tube and a second sub-standard representing gamma rays generated by neutrons that thermalized in the surrounding borehole or rock formation. 
     
     
       8. The method of  claim 5 , wherein the ratio of contributions from the sub-standards is constrained based on environmental dependencies including one or more of the following: borehole size, formation density, borehole fluid density, porosity, hydrogen index of the formation, neutron slowing-down length of the formation, thermal neutron capture cross section of the borehole fluid. 
     
     
       9. The method of  claim 4 , wherein each of the at least two sub-standards includes different combinations of at least two elements found within the nuclear spectroscopy tool. 
     
     
       10. The method of  claim 9 , wherein each of the at least two sub-standards include different combinations of Fe, Cr, Ni, Mn, W, Co and Mo. 
     
     
       11. The method of  claim 1 , wherein each of the at least two sub-standards represent a common single element and are differentiated by region of origin in the formation, the borehole, or the nuclear spectroscopy tool. 
     
     
       12. The method of  claim 11 , wherein differentiating comprises comparing gamma ray attenuation represented by each of the at least two sub-standards. 
     
     
       13. The method of  claim 11 , wherein the standard spectra comprise one nominal spectral standard and one or more “difference” standards that represent the difference between the nominal standard and the remaining plurality of sub-standards. 
     
     
       14. The method of  claim 11 , wherein the standard spectra are derived such that a linear combination thereof provides an approximation of an overall spectral shape for the single element. 
     
     
       15. The method of  claim 11 , wherein the single element is hydrogen, and the sub-standards separately represent hydrogen in the formation and hydrogen in the borehole mud. 
     
     
       16. The method of  claim 11 , wherein the single element is chlorine, and the sub-standards separately represent chlorine in the formation and chlorine in the borehole mud. 
     
     
       17. The method of  claim 1 , wherein estimating an aspect of the formation comprises estimating a composition of the formation. 
     
     
       18. A method for estimating an aspect of a formation using a nuclear spectroscopy tool, comprising:
 placing a nuclear spectroscopy tool including a neutron source and at least one detector for gamma rays into a borehole; 
 emitting neutrons from the nuclear spectroscopy tool into the formation adjacent the tool such that some of the neutrons can generate gamma rays from the formation, some of the neutrons can generate gamma rays from an element in the nuclear spectroscopy tool, and some of the neutrons can generate gamma rays from an element in a drilling mud; 
 detecting with the tool an energy spectrum of gamma rays induced by the emitted neutrons; 
 analyzing the detected gamma ray spectrum using a combination of standard spectra including at least two sub-standards that represent a common element or group of elements and that are differentiated based on distinct locations of neutron interaction; 
 
       and
 estimating an aspect of the formation using the analyzed detected gamma ray spectrum, further comprising using one or more of Monte Carlo modeling and experimental measurements designed to isolate the gamma rays originating in various spatial regions to create at least one of the stored standard spectra. 
 
     
     
       19. A nuclear spectroscopy tool for estimating an aspect of a formation, comprising:
 a neutron source configured to emit neutrons into the formation adjacent the tool such that some of the neutrons can generate gamma rays from the formation, some of the neutrons can generate gamma rays from an element in the nuclear spectroscopy tool, and some of the neutrons can generate gamma rays from an element in a drilling mud; 
 a gamma ray detector configured to detect an energy spectrum of gamma rays induced by the emitted neutrons; and 
 data processing circuitry that carries out analysis of the detected gamma ray spectrum using a combination of standard spectra including at least two sub-standards that represent a common element or group of elements and that are differentiated based on distinct locations of neutron interaction, and estimates an aspect of the formation using the analysis, 
 
       wherein the nuclear spectroscopy tool comprises a spatial heterogeneity in its material composition and wherein the nuclear spectroscopy tool comprises a wireline tool having a bow spring extending through or along the wireline tool or a logging-while-drilling tool that is configured to accommodate passage of drilling mud.